Diverse Members of the Phylum Armatimonadota Promote the Growth of Aquatic Plants, Duckweeds

Duckweeds are small, fast-growing aquatic plants with high starch and protein content, making them promising candidates for next-generation plant biomass resources. Despite their importance, little is known about their interactions with microorganisms, particularly plant growth-promoting bacteria (PGPB), which play key roles in enhancing plant productivity. In this study, we report the plant growth-promoting effects of strain LA-C6, a member of the phylum <i>Armatimonadota</i>, isolated from duckweed fronds. Based on 16S rRNA gene analysis, this strain represents a novel genus-level lineage, and is referred to as <i>Fimbriimonadaceae</i> bacterium strain LA-C6. In axenic co-culture experiments, strain LA-C6 promoted duckweed growth, increasing the frond proliferation of four duckweed species (<i>Lemna minor</i>, <i>Lemna aequinoctialis</i>, <i>Spirodela polyrhiza</i>, and <i>Landoltia punctata</i>) by 1.8- to 4.0-fold compared with uninoculated controls. Importantly, three other phylogenetically distinct <i>Armatimonadota</i> species also exhibited significant plant growth-promoting effects on <i>L. minor</i>, increasing frond number by up to 2.3-fold and dry weight by up to 2.4-fold. This finding highlights the broader potential of diverse <i>Armatimonadota</i> members as PGP bacteria. A survey of the IMNGS database showed that strain LA-C6 and other <i>Armatimonadota</i> species are widely distributed across diverse plant-associated environments. Biochemical assays and gene prediction analyses revealed that strain LA-C6 produces indole-3-acetic acid (IAA) as a representative PGP trait, whereas no additional PGP-associated traits were detected. These results suggest that diverse bacterial lineages within the phylum <i>Armatimonadota</i> exert growth-promoting effects on aquatic plants, potentially through yet-to-be-identified mechanisms.